Maybe this is a sophomoric assumption, but I figured that
as long as there are 15-20 total changes of water, I'd be
good. What I currently do, then, when using a inversion
tank to develop film is:

I empty the tank of fixer - I fill the tank up completely
with water once and dump it - I fill the tank up with
water to just barely cover the developing reels and
dump it - I repeat the "fill to the reels" step 15
times - I repeat the "fill to the reels" step -
twice more with distilled water

Unbelievable. Extremely wasteful of water and time.
I suggest you and the lab adopt the Ilford sequence
or something similar for a wash routine.

I perform the sequence somewhat leisurely allowing
more time for the chemistry to diffuse outwardly.
So little water is used that room temperature
distilled does the job.

Same for prints; still water soaks with print
separators saves a Lot of water. Dan